Amey chooses polystyrene blocks for Scottish tunnel infill

Amey recently completed an infill project to make safe a disused railway tunnel underneath the approach roads north of Scotland’s Forth Road Bridge. The 420m tunnel was part of the Dunfermline to North Queensferry railway line that provided a link to the ferry service until the opening of the Forth Bridge in 1890. The 4.3m-wide and 5.1m-high tunnel with vaulted roof and brick lining continued in use for freight until 1954. The tunnel runs underneath the A9000 and B981 on the northern approach to the Forth
June 25, 2018 Read time: 3 mins
Amey made light of the project by using expanded polystyrene (EPS) blocks
Amey recently completed an infill project to make safe a disused railway tunnel underneath the approach roads north of Scotland’s Forth Road Bridge.


The 420m tunnel was part of the Dunfermline to North Queensferry railway line that provided a link to the ferry service until the opening of the Forth Bridge in 1890. The 4.3m-wide and 5.1m-high tunnel with vaulted roof and brick lining continued in use for freight until 1954.

The tunnel runs underneath the A9000 and B981 on the northern approach to the Forth Road Bridge. Both tunnel ends had been sealed and the adjacent cuttings filled in, leaving a vertical shaft at each end as the only access points.

An 2958 Amey inspection had found parts of the tunnel degrading and in need of preventative maintenance to ensure continuing structural integrity. Due to the limited depth of cover above the tunnel, a failure could have had an impact on the roads overhead.

Two options were considered: an ongoing programme of inspection and maintenance or a one-off project to infill the tunnel with a low-cost material. The infill option was chosen because it would eliminate the need for future inspections or maintenance making it the more cost-effective solution in the long term.

After considering workforce safety, overall cost and the need to avoid disruption to the local community, it was decided to fill the tunnel with expanded polystyrene (EPS) blocks. These were manufactured to a specific compressive strength capable of resisting the weight of rock and tunnel lining in the event of a localised structural failure.

The EPS blocks were pre-cut to a size and weight to facilitate easy manual handling on-site. This allowed work to be carried out from the access shaft at the north end of the tunnel which would keep construction traffic out of the village of North Queensferry for the majority of the work.

Another advantage of the lightweight blocks was that they could be delivered in large lorryloads, significantly reducing the number of required vehicle movements.

Once offloaded, the blocks were passed down the access shaft and transported along the tunnel to the work face hooked onto a specially designed sliding monorail system.

The tunnel was first lined with a hydrocarbon-resistant membrane. Then 21,342 EPS blocks were literally walked into place and arranged to allow safe working at height.

Once the tunnel was infilled, the access shafts were filled with concrete to seal the tunnel and prevent damage to the blocks.

The job was completed by late March, said Mark Arndt, Amey’s operating company representative for the Forth Bridges Unit. “This has been an unusual project,” he said. “It’s a real measure of success that most local residents were not even aware this work was taking place, despite the tunnel emerging within metres of homes in North Queensferry.”

Before sealing the access shafts, local primary schoolchildren filled two time capsules with items of their choice which were then buried among the blocks. Unlike concrete or aggregate material, EPS blocks are easily removed if the tunnel needs to be reopened.

Road Structures

For more information on companies in this article

Related Content

  • Europe closes in on the crossings
    September 27, 2017
    The Mersey Gateway bridge project off England’s west coast passed a milestone recently with the first joining of two of the deck sections. The key segments, as the sections are called, link the north approach viaduct to the north pylon deck span and are the first of four deck-joins scheduled for this summer. In total, there are five sections of bridge deck and approach roads that need to be joined.
    Read More
  • The Lessons of the Genoa bridge collapse
    April 23, 2019
    The partial collapse of the Polcevera viaduct, better known as the Morandi Bridge, has prompted debate regarding the technical and administrative aspects of maintaining road infrastructures. We discussed it with the engineer Gabriele Camomilla, former Director of Research and Maintenance of the Società Autostrade, who coordinated the only major structural intervention performed on the bridge, carried out in the early 1990s
    Read More
  • Scottish mountain highway toast to SuDS
    June 24, 2013
    Highway degradation and surface water pollution in an environmentally sensitive area of the Scottish Cairngorms Mountains, famous for its whisky production, are said to have been tackled with an innovative SuDS Solution from Hydro International. Poor drainage, flooding and freezing weather had led to a landslip and extreme surface degradation along a section of the narrow A95 that descends on a 10% gradient with tight bends to the beautiful village of Craigellachie on the River Spey, near Elgin. Situated im
    Read More
  • Scotland’s new Queensferry Crossing over the Forth Estuary
    December 23, 2015
    The new Queensferry Crossing under construction in Scotland will be the third landmark bridge spanning the Forth Estuary - Mike Woof writes When the new Queensferry Crossing over the Forth Estuary opens at the end of 2016, it will be the third landmark bridge to be built spanning this short stretch of water. Lying alongside the existing road bridge and the historic rail bridge, this new structure will be as groundbreaking as the two earlier crossings were at the time of their construction.
    Read More

Highlighted Content